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Climate change may be triggering rain of rocks in French Alps

By Fred Pearce

Time to take a different route

(Image: Pierre Clatot/AFP/Getty)

They call it the “corridor of death” – and climate change may be making it worse. Climbers have long feared that rising summer temperatures are triggering rockfalls on the slopes of Mont Blanc in the French Alps. Boulders regularly rain down on the most popular route up Europe’s highest mountain, the Gouter corridor, during the summer. In late August 2011, guides closed the route after an especially large fall.

A study of 533 rockfalls reported by climbers since 2003, or seen on satellite images of the Mont Blanc massif – an area of some 350 square kilometres – has now strengthened suspicions about the effects of warmer conditions.

Ludovic Ranavel of the University of Savoie at Le Bourget-du-Lac in the French Alps has identified a strong correlation between the number of rockfalls each year and air and rock temperatures high up the mountain.

Ravanel’s analysis reveals that 95 per cent of rockfalls around Mont Blanc happen during the hottest months, between June and September. Most begin in areas about 3100 metres above sea level, which he says is close to the bottom of the zone containing permanently frozen ice. As the Alps warm, this zone is receding and unleashing rocks on people below, he says.

“Cracks filled with ice are common in high mountain rock walls,” Ranavel says. Falls typically happen when rock temperatures in summer are between zero and -5 ºC. And after falls, seeping water can often be seen in the rocks behind.

In a previous study of large rockfalls identified in photographs of Mont Blanc and its surroundings over the past 150 years, Ravanel showed that these happened more often during unusually warm summers such as 1947, 1976, 1983 and 2003, and a strong general upward trend in the last two decades.

Wider problem

The problem is not restricted to Mont Blanc, or even the Alps. Summer rockfalls in regions with melting permafrost are a growing problem for mountaineers and those who live in these areas around the world.

In September 2002, 140 people died in the Caucasus mountains in North Ossetia, Russia, when some 100 million cubic metres of rock and ice hurtled down the northern slope of the Kazbek massif. Christian Huggel at the University of Zurich in Switzerland subsequently blamed the disaster on melting permafrost in rocks during high summer temperatures.

As permafrost melts in mountains around the world, the hazard from rockfalls should significantly grow this century, says Ravanel, and there may be many more corridors of death.

The rockfall data in Ravanel’s study is “unprecedented and really very valuable”, says Simon Allen of the University of Zurich.

“Similar clustering of rockfalls in the same temperature range has been observed in the Swiss Alps, and in southern New Zealand, and such processes are surely going on in other mountain regions of the world,” he adds. “But even very large rockfalls tend to go unnoticed and unrecorded in less visited regions.”

One thing’s for sure, says Allen. “We can expect more as temperatures increase.”